AP-endonuclease (APE) plays a central role in repair of most genomic damage induced endogenously and by environmental agents that induce oxidative stress. The mammalian APE, (APE1/Ref-1) additionally functions in transcriptional regulation: both as a reductive activator of transcription factors involving Cys 65 (or possibly another Cys residue) and directly, as a trans-acting co-factor (in acetylated form) in repressing negative Ca2+ response element (nCaRE)-dependent parathyroid, renin, and possibly other genes. Acetylated APE1 also binds to shear stress response element (SSRE), present in PDGF, eNOS and other shear-activated genes in endothelial cells. APE1 does not directly bind to a cis element, but to other proteins present in trans-acting complexes. Furthermore, APE1's stable binding to the Y-box specific trans-acting factor YB-1 suggests its role in regulation of even other genes including p53. We have generated conditional mouse embryo fibroblasts (MEF) lacking endogenous APE1 alleles but expressing human APE1 transgene;deletion of the transgene induces apoptosis of MEF, which showed that APE1 is essential for somatic cells as well as for the mouse embryo. Prevention of apoptosis by providing exogenous APE1, mutated to inactivate its repair function or its acetylation sites indicates essential roles of both functions. Regulatory activity of APE1 for cell survival is also suggested from its frequent overexpression in cancer cells. Furthermore, synergy between APE1 and p53 in spontaneous tumor induction in mice implicates APE1 in cancer prevention. In spite of extensive documentation of APE1's diverse regulatory functions, the requirement of its specific sequence motifs, domains and active sites for regulatory functions in various systems has not been clearly defined. Our central hypothesis is that APE1, in addition to carrying out essential repair of endogenous genome damage, regulates as a co-activator or co-repressor of genes which are essential for survival or for maintaining homeostasis after exposure to stress. To test this hypothesis and to elucidate APE1's structure- function requirements for the regulatory activities we will pursue three aims: (i) to define the requirements of APE1's specific residues and motifs for preventing apoptosis in MEF mutants by complementation;(2) to characterize APE1's interaction with other trans-acting factors present in nCaRE-, SSRE-, and Y-bound-box complexes chosen as prototype systems;and (3) to confirm the regulatory functions of APE1 for a few apoptosis-linked genes identified by preliminary gene chip screening. The results obtained from these studies will provide a comprehensive picture of the molecular bases for APE1's diverse regulatory activities. Furthermore, our studies should help identify APE1-regulated key signaling pathways in tumor induction and promotion which could be potential targets for cancer therapy.